Low profile spinal prosthesis incorporating a cannulated bone anchor having a deflectable post and a compound spinal rod

Abstract

A cannulated bone anchor comprising a self-centering ball-joint suitable for use in a spine stabilization prosthesis which supports the spine while providing for the preservation of spinal motion. The cannulated bone anchor has a deflectable post partially received in a socket of a housing formed in the head of the bone anchor. A centering rod received partially in the post and partially within the housing operates to align the post with the longitudinal axis of the bone anchor. Deflection of the post bends the centering rod which in turn applies a restoring force upon the post. The centering rod includes an inner-superelastic core and an outer polymer sheath.

1 . A cannulated spinal stabilization device comprising:
 a spiral shaped centering rod having a first end and a second end which the centering rod has a centering rod lumen;   a deflection post including at least a partial ball-shaped retainer and a rod;   a first bore extending along a longitudinal axis of the deflection post and opening through the ball-shaped retainer opposite the rod, wherein the first end of the centering rod is received in the first bore;   a housing;   a socket within the housing, wherein the socket partially encloses the ball-shaped retainer to form a ball-joint;   a channel extending from the socket out of the housing, wherein the rod of the deflection post extends through the channel out of the housing; and   a second bore in the housing extending from the socket opposite the channel, wherein the second end of the centering rod is received in the second bore;   a bone screw extending from said housing with the second bore extending through the bone screw;   whereby deflection of the deflection post bends the centering rod and the centering rod exerts a restoring force to center the rod of the deflection post within the channel; and   whereby the first bore communicates with the second bore such that the spinal stabilization device is cannulated.   
     
     
         2 . The spine stabilization device of  claim 1 , wherein the channel comprises a frusto-conical surface positioned to contact the rod after a predetermined amount of deflection of the rod from center of the channel. 
     
     
         3 . The spine stabilization device of  claim 1 , wherein said deflection post also includes a threaded mount external to the housing adapted to connect a spinal implant to the deflection post; and wherein the first bore extends through said threaded mount. 
     
     
         4 . The spine stabilization device of  claim 1 , wherein said deflection post is made of cobalt chrome alloy. 
     
     
         5 . The spine stabilization device of  claim 1 , wherein said centering rod is made of a super-elastic metal. 
     
     
         6 . The spine stabilization device of  claim 1 , wherein said centering rod is made of Nitinol. 
     
     
         7 . The spine stabilization device of  claim 1 , wherein the first bore further comprises an enlarged portion within the ball-shaped retainer, wherein the flexible section of the centering rod is received in the enlarged portion of the first bore. 
     
     
         8 . A cannulated spine stabilization device comprising:
 a first element including a rod with a first bore;   a second element including a bone screw with a second bore; and   a self-centering joint connecting the first element and the second element; said self-centering joint allows the rod to move relative to the bone screw and said self-centering joint including a spiral shaped centering rod with a third bore that communicates the first bore with the second bore so that the spine stabilization device is cannulated.   
     
     
         9 . The spine stabilization device of  claim 8 , wherein said self-centering joint is a self-centering ball-joint. 
     
     
         10 . The spine stabilization device of  claim 9 , wherein said self-centering ball-joint comprises a limit surface positioned to constrain movement of the self-centering ball-joint. 
     
     
         11 . The spine stabilization device of  claim 9 , wherein the self-centering ball-joint comprises:
 a housing having a socket;   a deflection post received in the socket; and   the centering rod received partially within the deflection post and partially within the housing;   whereby deflection of the deflection post bends the centering rod and the centering rod exerts a restoring force on the deflection post.   
     
     
         12 . The spine stabilization device of  claim 8 , wherein the rod of the first element is a mount adapted to connect to a spinal implant component and said first bore extends through said mount. 
     
     
         13 . A cannulated bone anchor comprising:
 a first element including a rod having a mount and a first bore extending through said rod;   a second element including a bone screw with a second bore extending through said bone screw;   a self-centering joint connecting the first element and the second element, said self-centering joint allows the rod to move relative to the bone screw; and said self-centering joint including a spiral shaped centering rod with a third bore that communicates the first bore with the second bore so that the bone anchor is cannulated.   
     
     
         14 . The cannulated bone anchor of  claim 13  wherein said centering rod includes a tube made of a super-elastic material and an outer sheath which covers said tube, which outer sheath includes a polymer. 
     
     
         15 . The cannulated bone anchor of  claim 14  wherein said super-elastic material is Nitinol and said polymer is PEEK. 
     
     
         16 . The cannulated bone anchor of  claim 13  wherein said self-centering joint includes a housing having a socket;
 wherein said rod includes a rounded end that is positioned in said socket; 
 said centering rod is received partially in said housing and partially in the rounded end of said rod; and 
 whereby deflection of the rod bends the centering rod and the centering rod exerts a restoring force on the rod. 
 
     
     
         17 . The cannulated spinal stabilization device of  claim 1  wherein said centering rod includes a tube with an outer sheath, which tube provides said first bore, which tube is made of a super-elastic material, and which outer sheath includes a polymer. 
     
     
         18 . The cannulated spinal stabilization device of  claim 17  wherein said super-elastic material is Nitinol and said polymer is PEEK. 
     
     
         19 . The cannulated spinal stabilization device of  claim 8  wherein centering rod includes a tube with an outer sheath, which tube provides said third bore, which tube is made of a super-elastic material, and which outer sheath includes a polymer. 
     
     
         20 . The cannulated spinal stabilization device of  claim 19  wherein said super-elastic material is Nitinol and said polymer is PEEK. 
     
     
         21 . The cannulated spinal stabilization device of  claim 1  wherein the spiral shaped centering rod is formed with a spiral cut. 
     
     
         22 . The cannulated spinal stabilization device of  claim 8  wherein the spiral shaped centering rod is formed with a spiral cut. 
     
     
         23 . The cannulated spinal stabilization device of  claim 13  wherein the spiral shaped centering rod is formed with a spiral cut.